Our long-term goal is to understand the mechanism and function of the ubiquitin- proteasome system in health and disease. This proposal focuses on the function of the XPC-Rad23 complex in DNA damage response and proteolysis. Mutations in XPC protein lead to a genetic disorder Xeroderma pigmentosum (XP) that exhibits extreme sunlight sensitivity and a strong propensity for skin cancer. XPC was known previously as a key DNA repair factor that cells employ to protect from sunlight-inflicted DNA injury. Specifically, XPC forms a complex with Rad23 and together scan the genome for DNA lesions, which can trigger nucleotide excision repair. However, not all of the phenotypes in XPC patients can be easily explained by DNA repair defects, suggesting that XPC may have other cellular roles. Others and we have previously found that its cofactor Rad23 participates in ubiquitin-mediated proteolysis. Rad23 specifically promotes the transfer of ubiquitylated substrates to the proteasome, which remains one of the most challenging issues in the ubiquitin field. Why do cells employ the same XPC-Rad23 complex in two distinct processes: DNA repair and proteolysis? We hypothesize that XPC and Rad23 functionally coordinate proteolysis and DNA damage response, and XPC mechanistically assists Rad23 in substrate selection. We have carved out unique niches and established solid footings to unravel the detailed mechanisms underlying the biological function of Rad23 and XPC in yeast and mammalian cells. We will tackle the elusive mechanism governing the dual role of XPC and Rad23 in proteolysis and DNA damage response via various state-of-art approaches including molecular biology, biochemistry, biophysics, genomics and proteomics. We propose the following aims in an effort to decipher the biological role of the XPC-Rad23 complex in substrate proteolysis. Aim 1 is to define the physiological role of Rad23 and XPC in mammalian cells. Aim 2 is to determine the biological function of Rad23 and Rad4 (yeast counterpart of XPC) in yeast. These studies should reveal novel insights into the mechanisms and functions of the ubiquitin system in DNA damage response. As ubiquitin plays pivotal regulatory functions in nearly every area of cell biology, the results will strongly impact other biological studies and provide defined molecular targets for future intervention in human diseases.